Cover as much as the top as possible. Using foil or plastic is good because you can press it firmly into place so it allows minimal amount of airflow around it. Be cautious with plastics or towels though if you have a Heat lamp or CHE. You don't want any fires. Seal as much of the top as possible. I got a small flower pot (4") and filled it with sphagnum moss and wet it really well. That helped mine a good bit. Sometimes adding a 2nd water bowl will help. As noted above, sponges are another great source. The more surface area you can have wet, the more evaporation and humidity you will likely get in return.